1. Field of the Invention
This invention relates to harmonic drive transmissions and more particularly to a supportive bearing arrangement on that harmonic drive transmission unit.
2. Prior Art
Harmonic drive transmissions were originally called "strain wave gearing", and were initially introduced by Musser in U.S. Pat. No. 2,906,143. Such original harmonic drive transmissions or "strain wave gearing" comprised a rigid, circular spline having "N" teeth. A flexspline having fewer than "N" teeth ("N" being a positive integer) and being disposed within the circular spline, and a rotatable wave generator disposed within the flexspline to deform the flexspline into a lobed configuration, such as an oval shape, so as to force the flexspline into engagement with the circular spline at two points on the major axis of the formed ovaloid.
The wave generator may include an oval cam plate and a bearing snuggly mounted on the outer periphery of the cam plate. The outer bearing is matingly inserted into the flex spline to as to deform it to the peripheral contour of the cam plate. An input shaft attached to the cam plate provides rotation thereto, causing the ovaloid configuration of the flexspline to be correspondingly rotated. During such rotation, the circular spline is induced to rotate, relative to the flexspline, in an amount proportional to the difference in the number of teeth between the flexspline and the circular spline. When an output shaft is arranged on either the flexspline or the circular spline, that output shaft is rotated very slowly in comparison to its input shaft. Such harmonic drive, strain wave gearing, has been utilized in machinery requiring a high reduction ratio.
The flexsplines are generally cup shaped, having an open first end and a closed second end. The closed second end usually comprises a diaphragm having a central, generally circular boss thereon. The boss may comprise a thickened portion of the diaphragm, as for example may be seen in U.S. Pat. No. 5,269,202 to Kiyosawa et al. The diaphragm radially outwardly of the boss disclosed in this patent decreases in thickness as it extends radially outwardly from that boss. This flexspline will be bolted to a hub for transmission of rotary motion therebetween. A shaft is typically connected via a circular bolting pattern to the boss. The shaft itself is then supported within an annular bearing arrangement disposed between the shaft and an annular housing enclosing the flexspline cup and wave generator. The alignment requirements for such a bearing arrangement in conjunction with the attachment to the boss, requires high tolerance machining and extended assembly.
It is an object of the present invention, to provide a harmonic drive assembly which improves over the prior art.
It is a further object of the present invention, to provide a harmonic drive assembly which is less expensive to manufacture by virtue of fewer parts and by virtue of elimination of possible misalignment inherent in prior art devices.
It is yet a still further object of the present invention, to provide a harmonic drive assembly utilizing a bearing arrangement which minimizes assembly time and improves the performance of the harmonic drive unit.